In various manufacturing, construction, and medical industries, fasteners are utilized that are threaded or screwed into place. These fasteners may require a predetermined amount of torque that has been determined to be optimal for a given fastening situation. In addition, the fastener may identify or stipulate a predetermined amount of torque that has been determined to fatigue or break the fastener. Often, these predetermined torque values are determined by the manufacturer or by a testing facility. In use, a technician or user may employ a device such as a torque wrench to set the fastener according to the predetermined amount of torque. In a particular example, bone screws may be employed by surgeons to reconstruct bones or attach reconstructive components to bones of patients. In such circumstances, applying a proper amount of torque may be critically important.
Conventional torque wrenches utilize forces from coil springs and spring washers along with friction to limit the amount of torque applied. Unfortunately, as components within these conventional torque wrenches slide by one another, wear may alter the torque setting of the conventional torque wrench. As such, conventional torque wrenches often need to be re-calibrated to maintain their torque limit range, typically every six months.
In a typical example, conventional torque wrenches will utilize Belleville washers, which are slightly convex or domed, and flex under pressure. The precision of the force of these Belleville washers may be limited by their dimensional tolerances and their reported spring rates, which may vary from batch to batch. Because they may exhibit a high force value over a small travel distance, the use of many washers may be required to reach a travel distance necessary for the assembly to cycle. Thus, the washers may be stacked up along a rod and pressed together with a screw plunger. In some cases, the washers are alternatively flipped, back-to-back or front-to-front. The net result is a tool that may have many small, imprecisely-aligned pieces, wherein each part may wear and flex at different rates.
In many conventional or state of the art torque tools that can be classified as “screwdriver” type tools, i.e. a handle connected to a shaft, the torque-limiting cycle may be controlled by a “one way dog clutch”. This type of device is composed of two components, both aligned along a common axis. One component is stationary and the other is free to rotate about and slide along the axis. A series of radial ramps on each face align and lock the faces together. The rotation of the free component causes the two faces to alternately align and then slip over each other to the next alignment. During the rotation cycle, the faces separate by displacing a spring designed to apply a predetermined amount of torque resistance to the cam face engagement.
When the faces realign they do so rapidly with the force of the spring driving them together. This snapping together action is known to cause one or the other clutch component to split in half, thereby, breaking the clutch mechanism, additionally the spring washer stack is in constant compression and is continually pressed against a flat washer that in turn presses upon the dog clutch.
In the case of the wrench-style tool, the spring stack presses upon a plunger which, in turn, presses against one of the faces of a rotating cam. The plunger, in turn, pushes the cam into the opposing housing wall, thereby, creating the friction or resistance to rotation.
In axial handled torque-limiting tools, torque limiting occurs when the cam or clutch rotates by pushing the spring washers rearward a sufficient distance to allow rotation. The cam faces rub against each other with every rotation. When the cam rotates, the spring washers are compressed and the cam faces move apart due to the angular inclination of the ramped surfaces. Peak torque is reached when the cam faces are at their maximum separation. When the cam faces realign, they snap together into the next low torque position, and thus, no additional torque may be applied to the fastener.
A major disadvantage of conventional axial handled torque-limiting tools is that the spring washers are constantly under a compression load, even when the tool is at rest. Parts under load tend to fatigue more quickly than parts at rest; as a result, these types of tools require frequent recalibration, usually every six months. Furthermore, each snap of the cams coming together sends a shock wave down the tool shaft and into the fastener, thereby, transmitting an unnecessary force through the tool shaft to the fastener, this action, if of sufficient strength, could potentially damage the fastener.
In the conventional torque wrench type tool, a handle is attached to a tool head, which contains a hexagonal cam. In use, a tool shaft is inserted and locked into the center of the cam. The handle extending from the head contains a piston and spring washers or compression springs, which press the piston against one of the cam faces. The cam face is in turn pressed against the inner wall of the tool head. As with the axial handled torque-limiting tools, when the fastener begins to resist rotation and this resistive force exceeds the predetermined torque force of the springs, the shaft and cam may no longer be driven by the piston. Continued rotation of the handle may cause the piston to compress the springs, permitting the piston to slip over the cam peak to the next cam face. When the piston is located at the cam peak maximum torque is achieved and no further force may be applied to the fastener. With this design as well, the springs are under constant compression load leading to increased susceptibility to wear and breakage. The piston pressure causes the cam to rub and chafe against the opposite inner wall of the tool head during its rotation. Thus, heavy lubrication may be required. Also, the cam is not centered in the head and moves freely when the piston pressure is removed.
When conventional torque-limiting mechanisms fail they bind or lock-up, thereby losing the torque-limiting effect and essentially, converting the tool into a rigid, non-limiting tool, wherein the torque is regulated by the user's ability to discern torque forces by hand. This could lead to over-torqueing, which is an unsafe condition especially in the medical context.
Accordingly, it is desirable to provide a device that may be capable of overcoming at least to some extent the disadvantages of wear and breakage described herein.